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Unit 4: Secondary Storage Devices

data has been etched on the surface of a WORM disk it becomes permanent and can be read Notes
but never altered. However, all the data to be recorded on a WORM disk can be written on
its surface in multiple recording sessions. The session after the first one are always additive
and cannot alter the etched/burned information of earlier sessions. The information added
in a session can be hidden in a subsequent session by creating the File Allocation Table (FAT
at a new location but the etchings on the surface cannot be removed. Such disk is called
Multi-Session Disk. Laser beam technology (discussed earlier) is used for data recording/
reading.

CD Read/Write (CD-RW) Disk
A CD Read/Write (CD-RW) disk is very similar to a WORM disk with the exception that
you can erase the previous content and write on it multiple times. Such disks use metallic
alloy layer. Laser beam changes the chemical property during writing (or burn process)
changing reflectivity at desired places. The land-pit difference on CD-RW is not significant
and hence CD drives have to be compatible to read such disks. A CD-RW disk usually has
a lifetime of 100 or more erase-write cycle. A disk written once can be erased by changing
by the chemical property again and then it can be written on to afresh. CD-RW drives have
such erase capability. CD-RW disks are little expensive than CD-R disks but are a great cost
saver because they can be reused many times due to their erase capability.

Digital Video (or Versatile) Disk (DVD)
DVD was designed primarily to store and distribute movies. However, it is fast becoming
mainstream optical disk as prices are reducing and need for large capacity storage is increasing.
It is similar to CD-ROM in principle but is denser in recording data. It follows Eight-to-
Fourteen Modulation Plus %1Plus) encoding as compared to Eight-to-Fourteen Modulation
(EFM) encoding used by CD-ROM. There are variants of DVD—single-layer disk and double-
layer disk. Single-layer disk has storage capacity of 4.7GB, whereas, double-layer disk has
storage capacity of 8.5 GB.

• Like CD-ROM, DVD also has many types—DVD-R/RW, DVD+R/RW, DVD-Video,
and DVD-Audio. DVD-Video is now the most dominant movie storage format used.
It allows storage of video in 4:3 or 16:9 aspect ratios in MPEG-2 video format using
NTSC or PAL resolution. It may be of interest to note that NTSC has resolution of
720 x 576 and PAL has resolution of 720 x 576. The audio is usually Dolby Digital

(AC-3) or Digital Theater system (DTS) and can be either monaural or 5.1 Surround
Sound.
DVD has enough space to store movie and support multi-lingual subtitles, multi-lingual
audio, multiple camera, angles etc. It also supports region marking to protect against piracy
and use of DVD from one region to another. It also supports Content Protection for
Prerecorded Media (CPPM) security technique to safe guard against n; etc.

4.1.2.2 Advantages and Limitations of Optical Disks
Advantages
1. Cost-per-bit of storage for optical disks is very low because of their low cost and high
storage density. Additional cost benefit comes from the fact that some optical disks can
be erased and reused many times.

2. Use of a single spiral track makes optical disks an ideal storage medium for reading
large blocks of sequential data such as audio or video.

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